1. Field of the Invention
The present invention relates to a semiconductor optical device, in particular, the invention relates to a semiconductor light-emitting device.
2. Related Background Art
One type of semiconductor laser diodes with the buried hetero-structure, hereafter denoted as BH, has been well known in the field of the optical communication system as an optical signal source. The BH structure buries the mesa stripe including the active layer by the current blocking layers made of a semiconductor material in both sides of the mesa stripe. The current blocking layer usually has bandgap energy greater than that of a semiconductor material for the active layer. Comparing this BH structure with other types of the semiconductor layer diode, such as ridge waveguide type, the BH structure efficiently concentrates the carriers injected from the electrodes and also the light within the active layer, which may enhance the emission efficiency.
FIG. 6 shows an example of the BH structure. The laser diode, hereafter denoted as LD, 56 shown in FIG. 6 provides the mesa stripe between the cathode 101 and the anode 112. The mesa stripe includes the n-type lower cladding layer 103, the active layer 105, the p-type upper cladding layer 107, and the p-type contact layer 108 on the n-type semiconductor substrate 102. The current blocking layer 109 buries the mesa stripe in both sides thereof. The current blocking layer 109 is often doped with impurities that cause deep levels in the energy bandgap, typically, irons (Fe) are applied as the impurities. The deep levels may capture electrons and/or holes; accordingly, the current blocking layer 109 may operate as a semi-insulating layer. Because such a current blocking layer with the semi-insulating characteristics may reduce the parasitic capacitance of the LD 56, comparing with another type of the current blocking layer with the reversed biased p-n junction, the LD may operate in high frequencies over 10 GHz.
Several prior arts have reported the LD with the BH structure that provides the current blocking layer for burying the mesa stripe and doped with irons (Fe). It is also well known that impurities doped in the current blocking layer and other impurities doped in layers adjacent to the current blocking layer easily inter-diffuse. In particular, irons in the current blocking layer and zincs in the p-type cladding layer easily cause the inter-diffusion, in which the current blocking layer inter-diffused with Zn from the p-type cladding layer and provided with holes decreases its resistivity; while, the p-type cladding layer inter-diffused with Fe from the current blocking layer, which forms the deep levels to capture the carriers, increases its resistivity. Thus, the inter-diffusion of impurities degrades the current injection efficiency into the active layer in the mesa stripe, which increases the threshold current and decreases the emission efficiency of the laser emission.
Deep levels in the semiconductor layer generally capture only one of carriers, electrons or holes, that is, the semiconductor layer with the deep level generally shows the insulating characteristic only for one of the carriers. For instance, the current blocking layer doped with Fe can capture the electron but the hole; such a layer may show the high resistivity only for the electron. Therefore, when the Fe-doped layer comes in directly contact with a p-type layer, the electrons captured in the deep level in the Fe-doped layer and the holes diffused from the adjacent p-type layer and not captured by the deep level may easily recombine in the Fe-doped layer, which causes the leak current. Thus, the leak current flowing outside of the active layer reduces the carrier injection efficiency into the active layer, which degrades the device performance in, for instance, the threshold current and the slope efficiency.
One type of the LD 57, whose layer structure is illustrated in FIG. 7, provides a supplemental InP layer 111 with the n-type conduction in addition to the layers shown in FIG. 6 between the current blocking layer 119 doped with Fe and the cladding layer 117 doped with Zn. This additional layer 111 may suppress the inter-diffusion of Fe and Zn. This n-type InP layer 111 prevents the hole from injecting from the cladding layer 117 into the current blocking layer 119, which operates as a hole-capturing layer. Accordingly, the layer structure shown in FIG. 7 may effectively suppress the device performance due to the leak current by the inter-diffusion of impurities.
However, the active layer 105 shown in FIG. 7 comes in directly contact with the n-type additional layer 111, which permits the leak current flowing in the n-type additional layer 111 in both sides of the active layer 105. This leak current is due to the electrons, and because the electron shows a light effective mass, which is equivalent to a large mobility; the leak current tends to be large. Thus, even the layer structure shown in FIG. 7 is not enough to suppress the degradation of the device performance, such as the increase of the threshold current and the decrease of the slope efficiency.